The present invention relates to high damping polymer compositions with superior high-temperature stability, mechanical strength and moldability.
Free radical copolymerization of vinyl-substituted aromatic hydrocarbons and butadiene, vinyl-substituted aromatic hydrocarbons and maleic anhydride, R1R2ethylenes and maleic anhydride, and alkyl vinyl ethers and maleic anhydride are known. Further, imidization between a maleic anhydride and a mono-primary amine group is a commonly known chemical reaction.
Two or more polymers may be blended together to form a wide variety of random or structured morphologies to obtain products that potentially offer desirable combinations of characteristics. However, it may be difficult or even impossible in practice to achieve many potential combinations through simple blending because of some inherent and fundamental problems. Frequently, the two polymers are thermodynamically immiscible, which precludes generating a truly homogeneous product. While immiscibility may not be a problem since it may be desirable to have a two-phase structure, the situation at the interface between these two phases very often leads to problems. The typical case is one of high interfacial tension and poor adhesion between the two phases. This interfacial tension contributes, along with high viscosities, to the inherent difficulty of imparting the desired degree of dispersion to random mixtures and to their subsequent lack of stability, giving rise to gross separation or stratification during later processing or use. Poor adhesion leads, in part, to the very weak and brittle mechanical behavior often observed in dispersed blends and may render some highly structured morphologies impossible.
Adding a filler to a rubber matrix is a common practice for tire tread preparation. Most fillers function as mechanical enhancing agents, with applications for increasing strength and modulus of the polymer matrix to be filled. The characteristics which determine the properties a filler imparts to a rubber compound are particle size, surface area, structure, and surface activity. These principal characteristics of rubber fillers are interdependent in improving rubber properties. In considering fillers of adequately small particle size, reinforcement potential can be qualitatively considered as the product of surface area, surface activity, and persistent structure or anisometry.
The general influence of each of these three filler characteristics above on rubber properties can be summarized as follows: 1. Increasing surface area (decreasing particle size) gives lower resilience and higher Mooney viscosity, tensile strength, abrasion resistance, tear resistance, and hysteresis. 2. Increasing surface activity (including surface treatment) gives higher abrasion resistance, chemical adsorption or reaction, modulus, and hysteresis. 3. Increasing persistent structure/anisometry gives higher Mooney viscosity, modulus, and hysteresis, lower extrusion shrinkage, tear resistance, and resilience, and longer incorporation time.
In general terms, the effect of a filler on rubber physical properties can be related mainly to how many polymer chains are attached to the filler surface and how strongly they are attached. Filler surface area and activity are the main determinants, supplemented by structure.
Using a filler that has the correct combination of the above-mentioned properties to promote damping properties in a soft polymer composition is desirable. Currently, untreated precipitated silica has been increasingly used to replace carbon black as the filler for pneumatic tire treads with the attendent benefit of reduced rolling resistance. However, strong filler-filler interaction among silica particles makes the filler conglomerates hard to break down during mixing, and the resultant compound also shows a high hardness. Furthermore, poor filler-polymer interaction results in poor wear resistance for silica-filled tread in the absence of a proper coupling agent. Therefore silane coupling agents may be employed to improve both the processibility and wear resistance for silica-filled tread compounds.
To achieve an optimal material property improvement for a soft polymer gel composition, proper selection of filler is of key importance. Filler-filler interaction should not be so strong as to increase significantly the compound viscosity during mixing as well as the compound hardness of finished product, yet obvious reinforcement is desired. The filler and the polymer matrix should be compatible enough so that the filler can be incorporated and dispersed into the matrix easily and uniformly while minimally disturbing the structure desired in the original polymer composition. High tensile strength and increased damping properties at elevated temperatures properties are desired to give optimum function of the soft polymer gel composition. Importantly, and as suggested above, fillers work differently in various polymeric systems. In this regard, use of a filler in a polymer gel to provide high damping properties has proven especially difficult.
The present invention is a polymeric gel composition including a polymer having at least two different monomer units selected from a vinyl-substituted aromatic hydrocarbon, a R1R2ethylene, an alkyl vinyl ether, a maleimide, and a conjugated diene, a hydrophobically treated filler; and, optionally, an extender. If the polymer includes a maleimide, a maleated polyalkylene is included.
The present invention is directed to the use of copolymer gel filled with a hydrophobically treated, reinforcing filler to improve the tensile strength, tear strength, damping properties, and high-temperature compression set of these copolymers. The Shore A hardness of the present polymer gel compositions at room temperature is less than about 50, preferably less than about 20, and most preferably less than about 10.
Two broad classes of copolymers are used in this invention. The first is centipede polymers and the second is triblock copolymers. Centipede polymers are grafted polymer compositions of a maleated polyalkylene and a poly(alkenyl-co-maleimide). The alkenyl group in the centipede polymers can be a vinyl-substituted aromatic hydrocarbon, a R1R2ethylene, and/or an alkyl vinyl ether. The grafted centipede polymer is a thermoplastic, glass-like material that becomes a soft and rubber-like elastomer after being oil-extended.
The triblock copolymers contain at least two blocks of a vinyl-substituted aromatic hydrocarbon and at least one block of a hydrogenated conjugated diene. This triblock is then mixed with a non-aromatic oil to become a soft and rubber-like gel after being oil extended.
The following definitions apply hereinthroughout unless a contrary intention is expressly indicated:
xe2x80x9cVinyl aromatic hydrocarbonxe2x80x9d and xe2x80x9calkenyl benzenexe2x80x9d are used interchangeably;
xe2x80x9cMaleic anhydridexe2x80x9d encompasses dicarboxylic acids, including maleic anhydride, which can form a copolymer with an alkenyl benzene, an R1R2ethylene, or an alkyl vinyl ether, the copolymer having dicarboxylic acid units which are capable of reaction with an amine functional group;
xe2x80x9cMaleimidexe2x80x9d encompasses the reaction product of an amine and the dicarboxylic acids described above;
xe2x80x9cR1R2ethylenexe2x80x9d as used herein encompasses compounds of the general formula: 
where R1and R2 are the same or different substituents on the same or different carbon atoms of the ethylene group, and are independently H or substituted C1-C20 alkyl groups;
The general term xe2x80x9cpoly(alkenyl-co-maleimide)xe2x80x9d includes poly(alkenylbenzene-co-maleimide), poly(R1R2ethylene-co-maleimide), and poly(alkyl vinyl ether-co-maleimide); and
xe2x80x9cCentipede polymerxe2x80x9d refers to the first class of polymers listed above, and xe2x80x9ctriblock polymerxe2x80x9d refers to the second class of polymers listed above.